1. Field of the Invention
The present invention relates to a method of coating of mass-produced bulk goods and including loading the bulk goods into a rotatable drum through an opening formed in a tide surface of the drum, providing a coating apparatus having a spray gun and arrangeable outside of the rotatable drum, and applying the coating material to the bulk goods with the spray gun.
2. Description of the Prior Art
For protecting from corrosion of mass-produced parts made of steel such, e.g., bolts, nails, screws, etc., they are provided with a coating having a different thickness and formed, e.g., of zinc.
The application of coating can be effected, dependent on the corrosion-resistant requirements the products should meet, by using galvanization, mechanical plating, hot zinc galvanization, etc.
At present, the most widely used coating process is galvanization. For coating, the mass-produced parts in form of bulk goods all displaced, in slowly rotating drums, through different baths.
As a result of the necessary wet-chemical pretreatment and the following metal dissipation, and dependent on the composition of the electrolyte, acidic, alkaline, and as a result of water decomposition, hydrogen can be produced.
The hydrogen can diffuse into the parts and lead, in particular in high-strength parts, to brittle fractures, primarily, to embrittlement.
A cost-intensive malleablizing process, during which the hydrogen is partially removed from the parts, permits to partially compensate the effect of embrittlement.
By further development of the process of producing an electrolyte and by modifying coating apparatuses, the coating process became more ecologically friendly. However, the pollution still remains very high and the manufacturing costs also remain high.
U.S. Pat. No. 5,393,346 discloses a process of coating of mass-produced bulk goods according to which a fluid coating is applied to bulk goods, which are located in a rotatable drum, through an opening formed in the drum. Then, the fluid coating is dried by heating the bulk goods. As a result, a film is formed on separate parts which, however, does not penetrate sufficiently deep into recesses of the parts.
Accordingly, an object of the present invention is to provide an environmentally friendly and cost-effective process for coating mass-produced bulk goods and which would permit to prevent any damaging penetration of hydrogen in separate parts.
This and other objects of the present invention, which will become apparent hereinafter, are achieved by heating the coating material in the coating apparatus.
During coating, as a result of rotation of the drum, the bulk goods are constantly displaced, and separate parts do not remain in the same location for a relatively long time. The coating material is heated and atomized in a coating apparatus, which is located outside of the drum, and is then applied to the bulk goods displaceable in the drum. The coating material is applied to the bulk goods at a speed from, e.g., 30 m/sec to 850 m/sec. As a result of a continuous movement, separate parts are uniformly coated with the coating material. The thickness of the coating can be controlled by changing the process parameters, e.g., the durability of spraying, and by changing the rate of deposition. The coating material is brought to a molten condition by heating.
For applying the coating on the bulk goods, according to the present invention, all thermal processes, such as autogenous flame-spraying with use of wire, autogenous flame-spraying with powder, flame-spraying of synthetic materials, detonation spraying, arc flame spraying, high-speed flame-spraying, plasma-spraying, laser-spraying, and cold gas-spraying, can be used. As a coating material, wire-shaped material, e.g., zinc wire, powder material, e.g., zinc powder, alloys, plastics, ceramics, and the like can be used.
An intensive cooling of the heated and atomized coating material, which can result from a relatively large distance between the bulk goods and the region of the coating apparatus in which the coating material is atomized, can be prevented by preferably heating the coating material in the spray gun of the coating apparatus located in the vicinity of the bulk goods.
Preferably, the surfaces of separate parts of the bulk goods are roughened and activated before coating by jet-blasting. The roughing and activation of the surfaces of the separate parts lead to a better adhesion of the coating material that is applied to the bulk goods.
The jet-blasting is preferably conducted outside of the drum in compressed-air, sand-blasting, or centrifugal apparatuses with use of fine-grained steel medium that is applied to the bulk goods at a high speed. The parts can have, after being subjected, to jet-blasting, a roughness of 16-20 micron.
An autogenous coating results in a relatively rough coating surface. If necessary, the surfaces of the parts can be subjected, after coating to smoothing. During the smoothing process, the typical, for a coating process, roughness is evened and the coating layer is compressed. This improves the appearance of the coating and the corrosion-resistance characteristics.
The coated bulk goods, preferably, are subjected to smoothing outside of the drum in a smoothing apparatus with use of at least one auxiliary element. As an auxiliary element, small, polished steel balls having a diameter from, e.g., 0.3 mm to about 5 mm, can be used. The container of the smoothing apparatus can, during the smoothing process, rotate or wobble. The container can have a cylindrical shape and be provided with a conveying screw for transporting the to-be-smoothed bulk goods and the auxiliary element(s) from the entrance of the container to the opposite-end.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings.